1. Field of the Invention
The present invention generally relates to a method and apparatus for modifying a television signal to inhibit the unauthorized reproduction thereof by a video tape recorder (VTR) and, more particularly, to a method and apparatus for inhibiting the unauthorized reproduction of programming in a multiplexed analog component (MAC) television system.
2. General Background
U.S. Pat. No. 4,914,694 to Leonard describes a method and apparatus for modifying a composite television signal to inhibit the reproduction of an unauthorized recording thereof by conventional video tape recorders, but to enable the display of a video picture therefrom on a television receiver. The vertical period of a television signal is modified so as to increase or decrease the vertical period with respect to nominal field intervals of 16.683 milliseconds, thereby defeating the ability of virtually all commercially available videotape recorders to record and satisfactorily reproduce a video picture from the modified television signal. By adjusting the vertical period, either by maintaining a constant number of horizontal line intervals but varying the duration of groups of those line intervals, or by adding or deleting line intervals while maintaining a constant duration of each line interval, the capstan and drum servo units normally provided in VTRs are inhibited from operating satisfactorily. However, this vertical period adjustment does not prevent the vertical sync detecting circuitry typically utilized in most television receivers from displaying satisfactory video pictures. Thus, the modified television signal cannot be adequately recorded and reproduced, but nevertheless can be satisfactorily received for video picture display on a conventional television display.
FIG. 1 (corresponding to FIG. 1 of the Leonard '694 patent) illustrates the apparatus for implementing this technique. A received television signal, which may be supplied from a video recorder or from conventional television signal generating or transmitting apparatus, is digitized by A/D converter 102 to produce pixels having respective pixel values over the active video portion of each line interval. Successive lines of pixels in each received video field are written into a field memory included in memory 104 under the control of write control circuit 106. The pixels are written into the memory at a standard fixed rate synchronized with the normal horizontal synchronizing frequency f.sub.h. As one field of pixels is written into memory 104, a preceding field of pixels is read from the memory under the control of read control circuit 108. The output of memory 104 is coupled to D/A converter 112 which is adapted to convert the digitized pixel values to an analog signal. D/A converter 112 is coupled to a mixer 114 which is also coupled to a synchronizing signal generator 116. The mixer functions to insert the usual horizontal and vertical synchronizing signals, burst signals and equalizing pulses conventionally used in NTSC format as well as the non-active line intervals. The output of the mixer thus comprises the modified television signal containing the original video information, but with more or less lines per frame. Time code reader/generator 122 serves to supply processor 110 with an indentification of each frame in a received television signal. This frame identification information is used by processor 110 in conjunction with profile data retrieved from profile library 118 to control the reading out of line intervals from memory 104. In order to minimize perturbations and interference in the displayed picture, changes in lengths of frames pass through standard lengths at scene changes. For this reason, scene change detector 120 is coupled to processor 110 to apprise the processor of the particular frame in which a scene change is detected. A monitor 126 is coupled to receive and display the television signal and a supervisory control 128 is coupled to processor 110 to permit a supervisor to supply a signal to the processor stopping continued changes in the vertical period.
To vary the number of lines per frame, the rate at which line intervals of pixels are read from memory 104 remains constant. A profile pattern stored in profile library 118 establishes the number of lines includes in each frame read from memory 104, and processor 110 advantageously varies the start time at which the first line of active video information is read from memory 104 by read control circuit 108.
In the event that the profile pattern calls for the number of lines included in a frame to be greater than the standard number, processor 110 commands synchronizing signal generator 116 to continue to generate non-active (or "black") horizontal line intervals which are supplied by mixer 114 as the output TV signal. The processor also commands read control circuit 108 to delay the time at which the stored lines of active video information are read from the memory. Hence although the same number of active lines are included in the output TV signal, the total number of lines therein is greater than the standard number because synchronizing signal generator 116 supplies "extra" black lines. Alternatively, if less than the standard number of lines is to be included in a frame, thereby reducing the frame length, processor 110 interrupts the generation of black horizontal line intervals by sychronizing signal generator 116, and concurrently advances the time at which read control circuit 108 reads the stored lines of active video information from memory 104.
U.S. Pat. Nos. 5,003,590; 4,439,785; 4,673,981; 4,390,898; and 4,488,176 disclose methods and apparatus for preventing unauthorized taping of programming and are incorporated herein by reference.
While this system has been utilized in NTSC and other composite television systems, other signal types may, for example, be utilized in satellite television systems. A MAC color television signal is illustrated in FIG. 2, which is an amplitude-vs.-time diagram of a single video line of 63.56 microseconds duration. The first 10.9 microseconds is the horizontal blanking interval (HBI) 22, in which no picture information is transmitted. Following HBI 22 are chrominance signal 24 and luminance signal 26, either of which may be time-compressed. Between chrominance signal 24 and luminance signal 26 is a 0.28 microsecond guard band 28, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 2 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active video line, for reading, thus creating the MAC signal of FIG. 2. Audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The single rate at which all samples occur in the MAC signal is called the MAC sampling frequency.
As is conventional in television, thirty "frames" each comprising a still image are transmitted per second. Each frame includes two "fields". In a preferred embodiment of the invention, the video encoding scheme employed is that referred to generally as "B-MAC." This is an acronym for type B format, Multiplexed Analog Component system. "Type B" refers to the fact that data is carried integral to the video signal. See generally Lowry, "B-MAC: An Optimum Format for Satellite Television Transmission," SMPTE Journal, November 1984, pp. 1034-1043, which discusses in detail the B-MAC format and explains why it was chosen over various competing systems and is incorporated herein by reference.
The vertical blanking interval (VBI) of each field contains certain "system data" necessary for operation of a satellite television system, as well as addressed packets and teletext lines used to carry data needed for the operation of individual decoders and for transmission of messages to individual subscribers. Preferably, the vertical blanking intervals of sixteen total fields are used for complete transmission of all system data required, which includes an encryption key which is changed every sixteen fields, that is, on the order of three times per second. Each line also includes a horizontal blanking interval (HBI). During the HBI are transmitted six channels of high quality digitally-encoded audio information, with error correction, such that the decoder can also be used to supply a high quality audio signal. This can be used to provide the audio component of the corresponding video signal (or several versions thereof, in different languages) or an additional audio signal, such that subscription audio is also made available according to the system of the invention.
FIG. 3 shows the format of the horizontal blanking interval (HBI). Preferably, the HBI consists of 78 total bits of pulse amplitude modulated (PAM) data. The HBI is interposed between vertical blanking interval or video information from a previous line and that of the present line. A typical horizontal blanking interval as shown begins with a two-bit guard band 30, followed by 45 bits of audio and utility data 32, a second two-bit guard band 34, twenty bits of color burst information 36, a further guard band 38, six more bits of data 40 and a final guard band 42, after which the VBI or the video signal of the particular frame commences. The position of the color burst 36 within the HBI varies, to provide signal scrambling.
FIG. 4 shows some additional details of the horizontal blanking interval data 32 and 40 shown in FIG. 3. In the example shown, fifty-one total bits of data are provided in each line of the HBI, and each bit is four-level FSK encoded, such that each bit period includes transmission of two bits. One bit can be referred to as sign and the other as magnitude as indicated on FIG. 4. As shown, the first seventy-eight bits are digital audio; thus each frame provides a thirteen-bit digital representation of a sample of each of six audio channels. High quality transmission of audio frequencies up to approximately 15 kHz is thus provided. Following the audio information are six bits of stepsize and bandwidth information. The stepsize bits indicate the size of the steps numbered by the thirteen bits of information preceding, and the bandwidth information relates to the amount of the amount of emphasis or de-emphasis of the signal employed. Alternate fields carry the stepsize and bandwidth data. Both these terms are used as conventional in the Dolby delta modulation scheme, which is employed in the preferred embodiment of this invention for transmission of the audio. Following are twelve bits of error correction code (ECC) for correction of the audio, indicated at 48. Four utility bits follow at 50, and the last bit 52 of the data are a parity check bits for checking the parity of the error correction bits 48.
FIG. 5 shows the arrangement of the lines which make up the vertical blanking interval (VBI). The VBI includes 16 lines in the 525-line NTSC version of this invention. A slightly different number of lines are used in the 625-line PAL embodiment of this invention. The functions of the lines and their arrangement in other respects are identical.
As indicated, the vertical blanking interval is 377 bits wide. Lines 1, 2 and 3 includes the transmission of clock recovery, synchronization and system service data, as indicated in FIG. 5.
Among the data contained in line 3 is a system key which is updated every sixteen fields, that is, which changes with each complete system data transmission. The system key is common to all decoders. The system key is contained in the system data of line 3, and is used for decryption of video program material, audio and teletext. The system key is preferably transmitted in encrypted form.
Lines 4-8 of the VBI include the addressed packets, as indicated by reference numeral 62. As noted, these each contain an address which is then followed by data, concluding with error correction coding (ECC). The addresses are those of the individual decoders. The addresses in the address packets are transmitted in the clear, such that they can be received without decryption by the receiver. The remainder of the message is preferably encrypted. Addressed packets addressed to differing decoders may be transmitted in a single field.
As indicated at 64, lines 9-13 of the VBI are used to transmit teletext. The first part of each teletext line is a teletext identification which indicates that the line in fact is teletext. As shown, two types of teletext lines are used. Teletext headers include a relatively larger number of flags, and indicate which of the following teletext lines are part of a particular "page" or message. The text lines themselves include a somewhat lesser number of flags and text data; typically forty ASCII-encoded bytes are sent per text line, and up to twenty lines can be displayed on the user's screen at once.
FIG. 6 shows in additional detail the clock recovery data of line 1 of the VBI. As indicated, its first portion 68 is the seventy-eight symbols of the HBI. Thereafter, line 1 includes a series of 1's and 0's which are used to synchronize the clock of the decoder.
FIG. 7 shows line 2 which is used for framing recovery, that is, for synchronization of the video signal. Again, the first portion 70 is the seventy-eight symbols of the HBI data; this is followed by framing recovery data, which consists of two repetitively transmitted eight bit sequences. One is the inverse of the other; the change from one to the other is made at the point marked "phase reversal." This line is used for framing recovery, i.e., for correct synchronization of the received video signal.
FIG. 8 shows in some additional detail the make-up of line 3. It begins with the seventy-eight symbols of HBI data indicated at 72, followed with a bit which is not used, and a number of message bits, each of which is immediately followed by a parity bit. The message bits shown in line 3 of FIG. 8 are each repeated three times and are each protected by parity bits, such that of some 378 total bits, only sixty-two bits of useful data are provided. This data comprises the "system data" used by the subscription television system of the invention to keep control of a wide variety of system functions. Three different versions of line 3 are required to transmit all the system data needed, and each is transmitted in five successive fields, such that the total system data transmission consumes fifteen total field transmissions. A sixteenth field is not used for transmission of system data. Most pertinent to the present invention is the fact that the system data transmitted in line 3 includes a service key which is changed every 16 frames, i.e., on the order of three times per second. This service key must, of course, be accurately received for the decoder to work properly. Therefore, it is transmitted redundantly, as outlined, and in combination with extensive parity-based error correction to ensure correct reception of the service key, as well as the other system data.
As discussed in Baylin et al., Ku-Band Satellite TV--Theory, Installation, and Repair, p. 122, a MAC system affords a number of advantages over a composite TV system. For example, time division multiplexing avoids any interaction between various signal components and can result in better quality reproduction. Color distortion is minimized and the available color bandwidth is increased.
Further, a B-MAC system is a conditional access system with scrambling, making it difficult for a subscriber to receive a program if he or she is not authorized. It is desirable to provide anti-taping in a system which not only has better color and video quality, but which also has protection against piracy. This could be accomplished by providing an anti-taping encoder at the location of B-MAC decoder. The video output by the anti-taping encoder could then include the variable frame lengths to prevent taping. However, to provide such an encoder requires a significant amount of video memory and it becomes prohibitively expensive to provide such encoders on a network-wide basis. It is also possible that a pirate could tap off the B-MAC decoder output before it enters the anti-taping encoder and thus compromise the effectiveness of the system.
Alternatively, encoding the control signals at the headend would be problematic because it disrupts field timing on normal MAC signals, and it is difficult if not impossible, for the decoder to maintain synchronization. That is, although a television is capable of synchronizing onto a signal having varying field length, a MAC decoder can not similarly synchronize onto a B-MAC signal having varying field lengths because of the differences in transmitting synchronization information. In MAC, one line of sync information is transmitted every field. Thus, there is only one reference per field. The varying field length makes it difficult for the decoder to determine where next field is going to start and the decoder almost immediately loses synchronization.
Stated differently synchronization recovery in B-MAC and many other scrambling systems such as VideoCipher is accomplished by means of a digital synchronization word transmitted once per field. Sync recovery must be extremely accurate and it is desirable not to dedicate a large portion of a channel to synchronization. When a prior art anti-taping system such as that described above is implemented, much stress is placed on sync recovery since the sync word is moved around.